In gas chromatography, it is often necessary to repeatedly inject a representative portion of a sample. The sample must be reproducibly introduced into a moving gas stream at pressures and temperatures well above ambient. Current injection systems consist of a hand held syringe or liquid sample valve which must deliver a reproducibly metered volume of sample in liquid form into an injection port or vaporizer. The injection port or vaporizing device volatilizes the liquid sample and entrains it in the flowing carrier gas stream which subsequently moves it to the separation column. The liquid must be volatilized in as short a time as possible; thus it is necessary to sufficiently heat the vaporization area and at times the carrier gas to rapidly flash the sample.
The hand syringe injection of a sample consists of first loading the syringe with a volume of sample by withdrawing the plunger. Next, the syringe needle is pushed through the septum, a small silicone rubber disc, of the injection port, and the plunger is depressed to discharge the sample by positive displacement into the injection port vaporization area. Finally, the syringe is withdrawn from the septum. The septum is the only barrier between the chromatographic process and the outside environment.
Where a liquid sample valve is used, the valve is generally heated and may be of the rotary, sliding plate or other types, consisting of a fixed volume sample loop or cavity through which liquid sample is generally flowing under pressure. When the valve is indexed to the sample inject position the liquid volume trapped in the sample loop or cavity is vaporized. Vaporization takes place by flashing the material in the carrier gas port of the heated sample valve or built in heated vaporizer while the material is exposed to a flowing carrier gas stream.
Both of the aforementioned systems are limited by temperature and to varying degrees by pressure, loss of light material or incomplete vaporization of heavy ends of the sample.
The hand syringe injection system, besides not being continuous, suffers from loss of light material present in full boiling range samples. The syringe and sample have to be heated at atmospheric pressure to a point where the material is fluid enough to load and displace from the syringe. Liquid in the needle tip may be prematurely forced out by thermal expansion when the needle enters the hot septum, thus depositing some material onto the septum which may later gradually release and interfere with the current and/or later sample results. Both of these effects can be observed when comparing the results of sampling with closed and open systems.
The septum must be changed daily because repeated puncturing will eventually destroy its mechanical strength which results in carrier gas leakage. If close attention is paid to the septum area upon withdrawal of the syringe, some vapors can be observed either exiting from the syringe needle or leaking momentarily from the septum, due to the pressure and high temperature required at the injection port to volatilize the liquid. This results in loss of material and varying sample volume.
An attractive feature of a syringe is the positive displacement of the liquid into the vaporization area, as compared to the heated sample valve. The sample valve relies on the sample loop or cavity to be at a sufficiently high enough temperature to flash volatilize the liquid so that it can be flushed out by the carrier gas. If the sample valve cannot be kept at a high enough temperature, some of the material, generally heavies, will remain in liquid form on the inner surface of the sample loop or cavity, thus preventing a correct analysis of the sample.
To minimize loss of light sample components and achieve repeatable sample volume injection the liquid sample valve with its metered flow through sample loop or cavity under pressure is definitely better than the hand held syringe. However, wider boiling range liquids require increased valve temperature to vaporize the liquid and increased pressure to maintain the light components as a liquid until time for injection. This prevents dual phase sampling which results in variable sample volumes.
The temperature and pressure at which a sample valve can be maintained on a continuous basis is limited by the valve's materials of construction. Present state of the art valves have a maximum continuous operating temperature of about 400.degree. F. at which point the seals begin to deform and the valves begin to leak. This limits the liquid sample which can be successfully injected to those with end points below 750.degree. F. Some manufacturers have valves they claim operate at about 660.degree. F. but investigation indicates that the valves do not operate on a continuous basis or for very long.
In the art of liquid chromatography, a syringe type injection system is disclosed in Oster et al. U.S. Pat. No. 3,631,724. The syringe is reciprocable in a block wherein it is engaged by a pair of spaced seals positioned to either side of a channel containing the carrier liquid. A portion of the syringe always remains in the carrier stream so as not to propagate a pressure surge. One end of the syringe has an opening at its end or in an adjacent portion of its side wall into which a measured dose of a liquid sample may be sucked by an internal piston when the syringe tip has a container of liquid brought into contact with it.